Reduced pitch laser redundancy fuse bank structure

ABSTRACT

A configuration for a laser fuse bank is disclosed wherein the available space is more efficiently used. Fuses of graduated width and variable configuration are placed so as to minimize the average distance between fuses and maximize fuse density. Alternatively, a common source is added to a standard laser fuse structure such that it intersects the fuses and the number of available fuses is doubled.

This application is a divisional of U.S. patent application Ser. No.08/577,468, filed Dec. 22, 1995. U.S. Pat. No. 5,636,172.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to integrated circuits and inparticular to a fuse bank structure used in integrated circuits.

BACKGROUND OF THE INVENTION

An integrated circuit is a complete electronic circuit, containingtransistors, diodes, resistors, and capacitors, along with theirinterconnecting electrical conductors, contained entirely within asingle chip of silicon. Integrated circuits continue to decrease insize, and the circuits they contain continue to increase in complexity.This increases the opportunity for defective chips resulting from afailed element or a defective conductor. The complexity of these devicesand the need to interconnect the circuits create very narrow performancetolerances. One way these needs have been met is to manufacture fusesinto the device. Fuses can be opened to isolate defective areas andallow the rest of the circuit to be used. Fuses can also be used to trima circuit, enable a particular mode, or enable or disable differentsegments of the circuit. By using fuses integrated circuit manufacturersare able to reduce the amount of semiconductor scrap. The continuousdrive to reduce the overall size of integrated circuits creates a needto design fuses and other elements of integrated circuits in such a wayas to minimize the space they require.

Another way to reduce semiconductor scrap is to provide redundantelements on integrated circuits. If a primary element is defective aredundant element can be substituted for that defective element. Oneexample of an integrated circuit device which uses redundant elements iselectronic memory. Typical memory circuits comprise millions ofequivalent memory cells arranged in addressable rows and columns. Byproviding redundant elements, defective memory cells can be replaced.Because the individual primary memory cells of a memory are separatelyaddressable, replacing a defective cell typically comprises openingfuse-type circuits to `program` a redundant cell to respond to theaddress of the defective primary cell. This process is very effectivefor permanently replacing defective primary memory cells.

Circuit designers continuously strive to achieve higher populationcapacities without a corresponding increase in physical size. Reducingthe size of individual elements in integrated circuits is one way inwhich available die real estate is maximized. For example, as memorydensity increases the number of fuses needed for redundancy in a givenmemory device also increases. A 256M DRAM is expected to have more than10,000 laser fuses. Most components of the memory devices can be scaledto meet the space restrictions resulting from the higher densities.However, laser fuses used to implement redundancy can not be scaled dueto mechanical restrictions related to current laser technology. Fusewidth must be kept large enough to cover the laser spot so that the fusecan absorb a large quantity of heat. In addition, the fuse-to-fuse spacemust be kept large enough to allow for mechanical laser alignmenttolerances and to prevent unintentional programming of a fuse adjacentto an exploding fuse. These laser alignment tolerances, as well as therequirements for a large passivation opening, limit the length of thefuse. Currently the constraints dictated by the laser repairrequirements limit the fuse pitch to about 3 microns. The demand forincreasing numbers of fuses combined with the fixed pitch limitationcreate a need for improvements in the laser fuses.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora design which provides an increased density of laser fuses.

SUMMARY OF THE INVENTION

The above mentioned problems with increasing the number of fuses in amemory device are addressed by the present invention which will beunderstood bad reading and studying the following specification. Adesign embodying a more efficient fuse shape is described which allowsplacement of more fuses in the same amount of physical space.

According to one embodiment of the present invention, an integratedcircuit laser fuse system is provided, comprising a plurality of laserfuse banks. Each laser fuse bank comprises three fuses. One of the fusesis a center fuse having a narrow end, a wide end, and a commoncenterline. Another of the fuses is a first outer fuse having a narrowend and a wide end, located adjacent to the center fuse such that thenarrow end of the first outer fuse is adjacent to the narrow end of thecenter fuse and the wide end of the first outer fuse is adjacent to thewide end of the center fuse, the wide end of the first outer fuse islaterally offset from the narrow end of the first outer fuse. Anotherfuse of the fuse bank is a second outer fuse having a narrow end and awide end, located adjacent to the center fuse and on the side of thecenter fuse opposite the first outer fuse such that the narrow end ofthe second outer fuse is adjacent to the narrow end of the center fuseand the wide end of the second outer fuse is adjacent to the wide end ofthe center fuse, and the wide end of the second outer fuse is laterallyoffset from the narrow end of the second outer fuse.

In one embodiment of the present invention the wide end of the firstouter fuse is laterally offset from its narrow end in an outwardlydirection away from the center fuse and the wide end of the second outerfuse is laterally offset from its narrow end in an outwardly directionaway from the center fuse. In another embodiment the plurality of laserfuse banks comprise a first laser fuse bank and a second laser fuse bankpositioned such that the second laser fuse bank is adjacent to the firstlaser fuse bank, and the second laser fuse bank is rotated one hundredeighty (180) degrees from the first laser fuse bank. In yet anotherembodiment the plurality of laser fuse banks are polysilicon fabricatedon an integrated circuit.

According to another embodiment of the present invention the pluralityof laser fuse banks are fabricated in a dynamic random access memory(DRAM). The DRAM comprises, in addition to the laser fuse banks, aplurality of primary memory cells and a plurality of redundant memorycells. The DRAM further includes a redundant enable circuit comprising alatch circuit coupled to one of the fuses of the plurality of laser fusebanks and a comparator circuit connected to the latch circuit and aplurality of external address inputs.

Yet another embodiment of the invention is an integrated circuit memorycomprising an array of primary memory cells arranged in rows andcolumns, a plurality of redundant memory cells, and a plurality of laserfuse banks.

In yet another embodiment of the present invention, an integrated laserfuse system is provided, comprising a plurality of substantiallystraight laser fuses, each one of the plurality of substantiallystraight laser fuses being located adjacent to and parallel withremaining ones of the plurality of substantially straight laser fuses,and a common ground connection interconnecting the midpoints of theplurality of laser fuses. In yet another embodiment the plurality oflaser fuse banks are polysilicon fabricated on an integrated circuit.According to another embodiment of the present invention the integratedlaser fuse system is fabricated in a dynamic random access memory (DRAM)comprising the laser fuse system, a plurality of primary memory cells, aplurality of redundant memory cells, a redundant enable circuitcomprising a latch circuit coupled to one of the fuses of the integratedlaser fuse system, and a comparator circuit connected to the latchcircuit and a plurality of external address inputs. Another embodimentof the invention is an integrated circuit memory comprising an array ofprimary memory cells arranged in rows and,columns, a plurality ofredundant memory cells, and the integrated laser fuse system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of an integrated circuit laser fuse bank asdisclosed in one embodiment of the present invention;

FIG. 1B is a block diagram showing the relative positioning of aplurality of integrated circuit laser fuse banks as disclosed in oneembodiment of the invention;

FIG. 2 is block diagram of a laser fuse bank as known in the art.

FIG. 3 is a block diagram of a common source laser fuse bank asdisclosed in one embodiment of the present invention;

FIG. 4 is block diagram of a conventional DRAM device; and

FIG. 5 is block diagram of a conventional fuse circuit.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent inventions. The following detailed description is, therefore,not to be taken in a limiting sense, and the scope of the presentinventions is defined only by the appended claims.

It will be understood that the following description of a DRAM isintended to provide a general understanding of the memory and is not acomplete description of all the elements and features of a DRAM.Further, the present invention is equally applicable to any size andtype of integrated circuit, including programmable logic,microprocessors, and memories, and is not intended to be limited to theDRAM described below.

One embodiment of the invention describes a pattern for efficientplacement of fuses and is illustrated in FIG. 1A. Shown is a fuse setcomprised of three fuses, hereinafter the left fuse 101, the centralfuse 102, and the right fuse 103. Each fuse has a narrow section 101a,102a, 103a and a wide section 101b, 102b, 103b. In this embodiment thesize and spacing of the narrow sections are set by the minimum spacingand conductor design rules, and the wider sections by the laser fusespacing and width design rules. The central fuse is comprised of anarrow and a wide section 102a, 102b which are connected end-to-end andhave a common center line 100. The wide section of the left and rightfuses 101b, 103b are connected to the narrow section of the respectivefuses 101a, 103a, but are laterally offset from the narrows section adistance determined by laser fuse spacing rules.

A fuse set is created by placing a left fuse, a central fuse, and aright fuse laterally parallel such that the narrow sections 101a, 102a,103a are adjacent and separated by the distance specified in the minimumspacing and conductor design rules. In one embodiment the minimumspacing and conductor design rules specify a pitch of 1.25μ and a widthof 0.5μ. The wide sections 101b, 102b, 103b are adjacent and separatedby the distance specified in the laser fuse spacing and width designrules. FIG. 1B shows how each succeeding fuse set is rotated 180° andplaced laterally parallel to the preceding fuse set such that theadjacent fuses of each set 103, 103' are separated by the distancespecified by the laser fuse spacing and width design rules. This patternis repeated to fill the available physical space. The fuses described inthis embodiment are preferably constructed of polysilicon, fabricated onthe top surface of an integrated circuit, and are programmed or "opened"by using a laser to evaporate a portion of the polysilicon. The fuse,therefore, normally has a conductive path from one end to the other. Byremoving a portion of the fuse, the conductive path can be opened.

Another embodiment of the invention is a fuse structure incorporating acommon source. FIG. 2 shows the conventional method whereinuniform-width fuses 201-204 are positioned parallel to one another usingthe laser fuse spacing and width design rules. There is a common ground210 on one end of the fuses. The other end of each fuse is connected toa fuse circuit, an example of which is discussed below. FIG. 3illustrates the positioning, according to the present invention, of acommon ground 210 perpendicular to the uniform-width fuses 201-204 andintersecting each of the fuses at their midpoint. Both ends of each fuseare connected to a fuse circuit. As can be seen, implementation of thisembodiment doubles the number of available fuses without increasing thespace requirements.

One example of how a semiconductor device employs fuses Such as thosedisclosed by the invention is shown in FIG. 4. The DRAM 400 of FIG. 4includes a DRAM array 402 which can be accessed by a microprocessor 404through input/output connections including address lines 406. The DRAMarray includes rows and columns of randomly addressable memoryregisters. The DRAM is accessed through address bus 406, row and columnaddress strobe signals RAS* and CAS*, write enable signal WE*, an outputenable signal OE* and by using other conventional control signals (notshown) which are known to one skilled in the art. Row addresslatch/buffer 408 and row decoder 410 receive and decode a row addressfrom a row address signal provided on address lines, and address acorresponding row of the DRAM array. Likewise, column addresslatch/buffer 412 and column decoder 414 receive and decode a columnaddress from a column address signal provided on address lines, andaddress the corresponding column of the DRAM array.

Data bus 416 receives memory register data during a write cycle from themicroprocessor for writing to DRAM array. Data stored in the DRAM can betransferred during a read cycle on bus 416. Control logic 418 is used tocontrol the many available functions of the DRAM. Various controlcircuits and signals not detailed herein initiate and synchronize theDRAM operation as known to those skilled in the art. Control circuitry418 can include redundant memory element enable circuits. That is, thememory array includes both primary and redundant memory cells. If aprimary memory cell is determined to be defective, a redundant memorycell can be programmed to function in place of the defective primarycell. The control circuit 418, therefore, controls both row decoder 410and column decoder 414 such that the proper memory cell is addressed.

Some of the inputs and outputs of DRAM 400 used to communicate withmicroprocessor 404 are described as follows. Write enable input (WE*) isused to select a read or write cycle when accessing the DRAM. To readthe DRAM array, the WE* line is high when CAS* falls. If the WE* line islow when CAS* falls, the DRAM is written to. Row address strobe nine rowaddress bid to clock in the nine row address bits and strobe for WE*.CAS*, and DQ. In standard memories, the RAS* also acts as the masterchip enable and must fall for the initiation of any DRAM array ortransfer operation. Column address strobe (CAS*) input is used to clockin the nine column address bits.

Address input lines are used to identify a row and column address toselect at least one memory cell out of the available memory cells ofDRAM array 402. DRAM data input/output lines 416 provide data input andoutput for the DRAM array. As stated above, the DRAM description hasbeen simplified for purposes of illustrating the present invention andis not intended to be a complete description of all the features of aDRAM.

One example of how a fuse circuit is employed in an electronic memory isillustrated in FIG. 5. Redundant enable circuit 500 is typicallyincluded in a memory, such as the DRAM described above. The redundantenable circuit can be included in control circuit 418 to access aredundant memory cell when an address of a defective primary cell isreceived on address lines 406. A comparator 560 is used to compare eachaddress line to reference line 570. Although only one reference line isillustrated, it will be understood that in a preferred embodiment eachaddress line has a corresponding reference line. Further, a redundantenable circuit 500 is provided for each redundant memory cell includedin the memory array.

To enable a redundant memory cell, fuse 540 is programmed, as describedabove, so that the reference line 570 matches the address of a defectivememory cell. That is, fuse 540 normally has a conductive path from oneend to the other. The input to inverter 550, therefore, is low and itsoutput 570 is high. When the fuse is programmed, the input to invertercan float unless latched to a proper voltage. To "read" the state of thefuse and latch the input of the inverter transistors 510, 520 and 530are provided. Providing a low signal to the gate of transistor 510during the memory circuit power-up sequence, transistor 510 is activatedand the source of transistor 520 is pulsed high to VCC. If fuse 540 isunprogrammed then line 580 is pulled low. Conversely, if fuse 540 isprogrammed the input to inverter 550 is pulled high through transistor590. The gate of transistor 530 is pulled low with the output ofinverter 550, which turns on transistor 530, thereby latching the inputof inverter 550 high. It will be understood that many different circuitscan be used to enable redundant memory elements and the presentinvention is not limited to the circuit described herein. Further,integrated circuits, including memories, can use the fuses of thepresent invention for any application where fuses may be needed.

Conclusion

A spatially optimized laser fuse bank has been described for use in anintegrated circuit. The pattern of fuses is used to increase the maximumnumber of fuses in integrated circuits, thereby allowing an increase inthe density of devices included in the circuit.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. An integrated circuit laser fuse systemcomprising:a plurality of laser fuse banks, each laser fuse bankcomprising:a center fuse having a narrow end, a wide end, and a commoncenterline; a first outer fuse having a narrow end and a wide end,located adjacent to the center fuse such that the narrow end of thefirst outer fuse is adjacent to the narrow end of the center fuse andthe wide end of the first outer fuse is adjacent to the wide end of thecenter fuse, the wide end of the first outer fuse is laterally offsetfrom the narrow end of the first outer fuse; and a second outer fusehaving a narrow end and a wide end, located adjacent to the center fuseand on the side of the center fuse opposite the first outer fuse suchthat the narrow end of the second outer fuse is adjacent to the narrowend of the center fuse and the wide end of the second outer fuse isadjacent to the wide end of the center fuse, and the wide end of thesecond outer fuse is laterally offset from the narrow end of the secondouter fuse.
 2. The integrated circuit laser fuse system of claim 1wherein the plurality of laser fuse banks comprise:a first laser fusebank; and a second laser fuse bank positioned such that the second laserfuse bank is adjacent to the first laser fuse bank, and the second laserfuse bank is rotated one hundred eighty (180) degrees from the firstlaser fuse bank.
 3. The integrated circuit laser fuse system of claim 1wherein the wide end of the first outer fuse is laterally offset fromits narrow end in an outwardly direction away from the center fuse. 4.The integrated circuit laser fuse system of claim 1 wherein the wide endof the second outer fuse is laterally offset from its narrow end in anoutwardly direction away from the center fuse.
 5. The integrated circuitlaser fuse system of claim 1 wherein the plurality of laser fuse banksare polysilicon fabricated on an integrated circuit.
 6. The integratedcircuit laser fuse system of claim 1 wherein the plurality of laser fusebanks are fabricated in a dynamic random access memory (DRAM), the DRAMcomprising:a plurality of primary memory cells; and a plurality ofredundant memory cells.
 7. The integrated circuit laser fuse system ofclaim 6 wherein the DRAM further includes a redundant enable circuitcomprising:a latch circuit coupled to one of the fuses of the pluralityof laser fuse banks; and a comparator circuit connected to the latchcircuit and a plurality of external address inputs.
 8. An integratedcircuit laser fuse system comprising:a plurality of substantiallystraight laser fuses, each one of the plurality of substantiallystraight laser fuses being located adjacent to and parallel withremaining ones of the plurality of substantially straight laser fuses;and a common ground connection interconnecting the midpoints of theplurality of laser fuses.
 9. The integrated circuit laser fuse system ofclaim 8 wherein the plurality of laser fuses are polysilicon fabricatedon an integrated circuit.
 10. The integrated circuit laser fuse systemof claim 8 wherein the plurality of laser fuses are fabricated in adynamic random access memory (DRAM), the DRAM comprising:a plurality ofprimary memory cells; and a plurality of redundant memory cells.
 11. Theintegrated circuit laser fuse system of claim 10 wherein the DRAMfurther includes a redundant enable circuit comprising:a latch circuitcoupled to one end of one of the plurality of laser fuses; and acomparator circuit connected to the latch circuit and a plurality ofexternal address inputs.